Yeah thats what dealerships and OEM taught you because they replace worn out parts rather cleaning on repairring them cuz more money 
Get hydrogen cleaning done. I got amazing results got civic 2011 cat con cleaned saw all gunk coming out. Before you criticise my knowledge trust me i did good 1 month research before getting it done i can give yoh link to.my posts where i posted all the data too. They cars avg went from 11 to 14km/l local as it was manual tranny.
This procedure is scotty kilmer approved too. You can youtube him. Also anyone who cleans cat con by taking it off or breaking its jaali is conning you. Get service done you will thank the information ive given youm jazkallah.